Origin of the vertically orientated clasts in brecciated shallow-marine limestones of the Chaomidian Formation (Furongian, Shandong Province, China)

摘要

Numerous shallow-marine limestone layers of the Furongian (Late Cambrian) Chaomidian Formation in the Jiulongshan section (Shandong Province, China) are breccias. Some of these breccias show abundant vertical to sub-vertical clasts. Typically, these clasts end abruptly at the contact plane with the overlying deposit, either abutting the overlying sedimentary bed or via an erosional plane that truncates the clasts. A few exposures show concentrations of clasts that must have been uplifted to the extent that they transgressed the then sedimentary surface or (possibly) penetrated the overlying sediment which, in this case, consists of muds or marls. The clasts tend to show clusters with respect to the enclosing fabric. All clasts are parallel to each other in a specific cluster, while the various clusters may show different orientations of the clasts. It is deduced that both the exceptional position and the exceptional orientation of the clasts must be ascribed to the upward movement of the clasts under the influence of pore water escaping under high pressure through fluidized sediment.